An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

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    they are suddenly transformed into singly refracting truly cubic crystals. In this case also there is a distinct contraction, and a considerable absorption of heat. Potassium sulphate, Kb SO 4, forms rhombic tables which are eminently pseudo-hexagonal, generally with a triplet structure which becomes more pronounced on heating, so that each crystal then appears to be composed entirely of fine lamellae parallel to {110} and {130}; simple crystals also give rise to twin lamellae on heating. Between 6oo° and 650°, according to Mallard, a sudden transformation takes place into a simple optically uniaxial crystal, whose double refraction is considerably stronger and of opposite sign ; this excludes the assumption that it is built up in a regular manner from sub-microscopic lamellae of the rhombic modification. Leucite, KAlSi2Oe, which is pseudo-cubic, behaves in a quite similar manner, and its transformation, at about 560°, into a truly cubic substance is apparently also a polymorphous change. Determinations of the thermal and volume relationships at the transition point have not been carried out on this substance, owing to the high temperature at which the change takes place. That, finally, one and the same substance can crystallise both in polysymmetric and in polymorphous modifications is indicated by the relationships which are observed in an important mineral group—that of the pyroxenes. The compound RSiOg, where R is a bivalent metal (Mg, Mn, Fe, etc.), exists (a) in monoclinic crystals, which often con¬ stitute lamellar twin structures of apparently rhombic form ; also {b) in rhombic crystals of precisely that form which would result from the above-mentioned twin lamellae becoming sub-microscopic, and lastly (c) in triclinic crystals of a distinctly divergent form which does not, by twin formation, exhibit gradations towards the two preceding forms. It is therefore to be assumed that the relation between the first and second forms is of the nature of poly¬ symmetry, whilst between these forms and the last one the relation is that of polymorphism, in this case dimorphism. A further example is supplied by the double sulphates KLiS04 and NH4LiS04. The first of these forms crystals, belong¬ ing to the hexagonal pyramidal class, which rotate the plane of polarisation of light, and whose structure may therefore